1. Field of the Invention
The present invention relates generally to a split gate type transistor and a method for manufacturing the same. Particularly, the present invention relates to a split gate type transistor employable as memory cells in non-volatile semiconductor memory devices and also to a method for manufacturing the same.
2. Description of the Related Art
Recently, non-volatile semiconductor memory devices such as ferro-electric Random Access Memory, Erasable and Programmable Read Only Memory (EPROM) and EEPROM have become increasingly popular. EPROMs and EEPROMs have a plurality of memory cells having floating gates and control gates. EEPROMs include flash EEPROMs which can perform data erasure for all of the memory cells or partial data erasure for each of a plurality of block memory cells. Split gate type cells and stacked gate type cells are present in the memory cells of the flash EEPROM.
In the flash EEPROM, the stacked gate type memory cell has no selector transistor, and thus it cannot make an ON-OFF selection by itself. Accordingly, when excess charges are extracted from the floating gate electrode in performing data erasure, a problem of excess erasure arises. For examples even if 0 V is applied to control gate electrodes so as to deenergize the memory cells, those memory cells which were subject to excess erasure are energized. Consequently, electric current constantly flows into the memory cells making it impossible to read data stored in the memory cells. In order to prevent such excess erasure from occurring, the erasing procedures in the memory device are preferably controlled precisely by a peripheral circuit or an external circuit.
Split gate type memory cells include a selector transistor which obviates the problem of excess erasure. International Patent Publication WO92/18980 discloses a flash EEPROM employing split gate type memory cells.
FIG. 1 is a schematic cross-sectional view of an example of a prior art split gate type memory cell 1. The split gate type memory cell (split gate type transistor) 1 has an N-type conductive source area 3 and an N-type conductive drain area 4 which are defined on a P-type conductive single crystal silicon substrate 2, a floating gate 7 disposed on a gate insulating film 6 on a channel 5 present between the source area 3 and the drain area 4, and a control gate electrode 9 disposed on an insulating film 19 and a tunnel insulating film 8 on the floating gate electrode 7. The arrow B in FIG. 1 shows movement of electrons from the floating gate 7 to the control gate electrode 9 in the erasure mode; while the arrow C shows movement of electrons from the channel area 5 to the floating gate electrode 7 in the write mode.
The insulating film 19 is formed with a Local Oxidation on Silicon (LOCOS) process. The floating gate 7 has lips 7b formed to extend upward from the upper edges thereof. The control gate electrode 9 has a first part serving as a selector gate 10 disposed on the channel 5 and the insulating films 6 and 8 and a second part disposed on the floating gate electrode 7 and the insulating films 6 and 8. The selector gate 10, the source area 3 and the drain area 4 comprise a selector transistor 11. Accordingly, the split gate type memory cell 1 has the transistor consisting of the floating gate electrode 7, the control gate electrode 9, the source area 3 and the drain area 4, connected in series with the selector transistor 11.
FIG. 2A is a schematic cross-sectional view showing a part of a memory cell array having a plurality of the split gate type memory cells 1, as disclosed in WO92/18980. The memory cells 1 are arranged in the form of a matrix on the silicon substrate 2. FIG. 2B is a schematic plan view showing a part of the memory cell array. FIG. 2A is a cross section taken along the line 2A--2A in FIG. 2B.
Each pair of adjacent memory cells 1a and 1b share the source area 3. The floating gate electrode 7 and the control gate electrode 9 of one memory cell 1a and those of the other memory cell 1b are arranged symmetrically on each side of the source area 3. Such symmetrical arrangement reduces the area to be occupied by the memory cells on the substrate 2. As shown in FIG. 2B, a field insulating film 13 for separating the memory cells 1 from one another is formed on the substrate 2. Each pair of memory cells 1a, 1b arranged in column also share the control gate electrode 9, with each control gate 9 forming a word line. The drain areas 4 in the pairs of memory cells 1a, 1b arranged in row are connected to a common bit line (not shown) via line contacts 14, respectively.
FIG. 3A is a schematic cross-sectional view showing a part of memory cell array having a plurality of split gate type memory cells disclosed in U.S. Pat. No. 5,029,130. Compared with FIG. 2A, the arrangement of the memory cells, the drain area 4 and the source area 3 are replaced with each other. FIG. 3B is a schematic plan view showing a part of the memory cell array.
A method for manufacturing a memory cell array 150 is described with reference to FIGS. 4A to 4G.
Step 1 (FIG. 4A): The silicon oxide gate insulating film 6 is formed on a device forming area of the substrate 2 according to the thermal oxidation method, and then a doped polysilicon film 31 is formed on the gate insulating film 6. A silicon nitride film 32 is formed over the entire surface of the doped polysilicon film 31 employing the Low Pressure Chemical Vapor Deposition (LPCVD) method. After a resist is applied over the entire surface of the silicon nitride film 32, the resist is partially removed by known photolithographic procedures to form an etching mask 33 for forming the floating gate 7.
Step 2 (FIG. 4B): After the silicon nitride film 32 is etched by means of anisotropic etching employing the etching mask 33, the etching mask 33 is removed. Next, the doped polysilicon film 31 is partly oxidized by means of LOCOS employing the partially etched silicon nitride film 32 as a mask to form the insulating film 19. In this process, the insulating film 19 intrudes into the edge portions of the silicon nitride film 32 to form bird's beaks 19a.
Step 3 (FIG. 4C): After the silicon nitride film 32 is removed, the doped polysilicon film 31 is etched by means of anisotropic etching employing the insulating film 19 as an etching mask. Thus, the doped polysilicon film 31 remaining after etching constitutes the floating gate electrodes 7. In this process, since the insulating film 19 has bird's beaks 19a formed thereon, the sharp lips 7b are formed along the upper edges of the floating gate 7 below the bird's beaks 19a.
Step 4 (FIG. 4D): The silicon oxide tunnel insulating film 8 is formed by thermal oxidation, LPCVD or a combination thereof over the entire surface of the device formed in Step 3.
Step 5 (FIG. 4E): A doped polysilicon film 34 is formed over the entire surface of the device formed in Step 4.
Step 6 (FIG. 4F): After a resist is applied over the entire surface of the device formed by in Step 5, an etching mask 35 is formed by means of photolithography.
Step 7 (FIG. 4G): The doped polysilicon film 34 is etched by anisotropic etching employing the etching mask 35 to form the control gate electrodes 9. Then, the etching mask 35 is removed.
As shown in FIG. 5, at the initial stage of forming the tunnel insulating film 8 in Step 4, an incomplete silicon oxide film 8a attributed to native oxide film, structural transition layer, etc. is formed. This incomplete silicon oxide film 8a contains not only silicon oxide having the complete O--Si--O bond but also dangling bonds not having the O--Si--O bond. While the process proceeds from Step 3 to Step 4, since lateral faces of the floating gate electrode 7 are exposed to the outside air, a native oxide film is formed on the surface of the lateral faces. The native oxide film contains dangling bonds not having the O--Si--O bond. Further, the structural transition layer is present at the border between the floating gate electrode 7 and the silicon oxide tunnel insulating film 8. In the structural transition layer, dangling bonds not having the O--Si--O bond are likely to exist.
FIG. 6 is a schematic cross-sectional view showing the memory cell 1 having an incomplete silicon oxide film 8a. The data stored in the memory cell 1 is erased when electrons in the floating gate electrode 7 flow to the control gate electrode 9 side as indicated by the arrow B. In this process, when the electrons accelerated by a high electric field pass through the tunnel insulating film 8 having such incomplete silicon oxide film 8a, the films 8, 8a undergo great stress. Accordingly, stress is repeatedly applied to the films 8, 8 by repeating a write operation and an erase operation to form and accumulate electron traps in the incomplete silicon oxide film 8a. In other words, an increase in the number of repeating write and erase operations (i.e. the number of data rewritings) increases electron traps. The thus formed electron traps inhibit migration of electrons from the floating gate electrode 7 to the control gate electrode 9. As a result, it becomes difficult to sufficiently withdraw the electrons in the floating gate electrode 7.
As described above, electron traps induce a phenomenon which is shown in FIG. 7, More specifically, in the read mode, although the electric current flowing into the memory cell (assuming the written state) is maintained at a constant level as shown, the current flowing into the memory cell 1 (assuming the erased state) attenuates, so that the difference between the electric current flowing into the memory cell 1 in the written state and the electric current flowing into the memory cell in the erased state becomes small as the number of data rewrites increases. This makes it difficult for a sense amplifier to read the data stored in the cell by detecting the level of the cell current. Thus, as it is difficult to read the data from the memory cell, the intended function of the memory cell cannot be expected. As described above, the incomplete silicon oxide film 8a makes it difficult to increase the number of data rewrites and also to reduce the operating life of the flash EEPROM.
Further, as shown in FIG. 8, in Step 4, when the tunnel insulating film 8 is formed according to the thermal oxidation method, the edges of the tunnel insulating film 8 intrude under the lower edges of the floating gate electrode 7 to form bird's beaks (gate bird's beaks) 8b. The bird's beaks 8b cause recession of the tunnel insulating film 8 on the opposite surface from the bird's beaks 8b to form gaps 8c. As shown in FIG. 9, in Step 5, the doped polysilicon film 34 intrudes into the gaps 8c to form sharp fins 9a corresponding to the profile of the gaps 8c along the lower edges of the control gate electrode 9.
The control gate electrode 9 permits, in the write mode, release of electrons from the fins 9a to induce a phenomenon called reverse tunneling such that electrons are injected from the control gate 9 into the floating gate 7, This reverse tunneling causes a problem in that data is written in memory cells 1 which were not selected for writing in the write mode. Consequently, it becomes impossible to write independent data in the respective memory cells 1, and the EEPROM cannot perform its intended function. As described above, the bird's beaks 8b cause reverse tunneling to make the flash EEPROM difficult to exhibit its function.